1. Field of the Invention
The invention relates to a transmitting optical element adapted for use in an objective in a projection exposure apparatus for microlithography and to an objective in a projection exposure apparatus for microlithography. The invention relates in particular to an objective having at least one optical element.
2. Prior Art
Projection exposure apparatus for microlithography are used to produce semiconductor devices and other finely structured components, such as integrated circuits or LCDs. Such a projection exposure apparatus contains not only a light source and an illumination system for illumination of a photomask or a reticle, but also a projection objective, which projects the pattern of the reticle onto a light-sensitive substrate, for example a silicon wafer which has been coated with a photoresist.
So far, three approaches in particular have been adopted in order to produce ever smaller structures in the order of magnitude of less than 100 nm: Firstly, attempts are made to enlarge to an ever greater extent the image-side numerical aperture NA of the projection objectives. Secondly, the wavelength of the illumination light is reduced ever further, preferably to wavelengths of below 250 nm, for example 248 nm, 193 nm, 157 nm or even less. Finally, further measures are used to improve the resolution, such as phase-shifting masks, multipole illumination or oblique illumination.
Another approach to increase the resolution capability is based on the idea of introducing an immersion liquid into the intermediate space which remains between the last optical element on the image side of the projection objective, in particular a lens, and the photoresist or another light-sensitive layer that is to be exposed. This technique is referred to as immersion lithography. Projection objectives which are designed for immersion operation are for this reason also referred to as immersion objectives.
The advantages of immersion lithography are due to the fact that the higher refractive index of the immersion liquid with respect to that of vacuum allows the illumination wavelength to be reduced to an effective illumination wavelength. This results in an increase in the resolution and the depth of focus.
When using immersion liquids with a high refractive index, a considerable increase of the incidence angle into the resist is possible in comparison to systems without immersion. This allows for a value of the numerical aperture (NA) of even more than 1.0. However, in order to allow maximum use of the advantage of high-refractive-index immersion liquids, it is necessary for the last optical element which is in contact with the immersion liquid to also have a high refractive index. In this case, “high” means a refractive index whose value at the given operating wavelength is considerably greater, that is to say by more than 10%, than that of quartz (n≈1.56 at λ=193 nm).
For optical elements in an objective in a projection exposure apparatus for microlithography at wavelengths of less than 250 nm either single crystal materials, such as calcium fluoride (CaF2) or quartz glass, are used as materials so far. However, quartz glass has not only a relatively low refractive index of 1.56 at a wavelength of 193 nm, but in addition qua glass has the disadvantage that local density changes occur when the UV radiation load is high, which lead to a deterioration in the imaging quality.
When using single crystal materials, such as CaF2, the problem of local density changes due to UV radiation does not occur. The refractive index of CaF2 at a wavelength of 193 nm is, however, only 1.5016. In addition, in CaF2 the effect of intrinsic birefringence becomes noticeable in this wavelength range and to an even greater extent at shorter operating wavelengths, such as 157 nm. The relationship between the refractive index and the polarization state of the incident light that is caused by the intrinsic birefringence restricts the imaging quality of the projection objectives produced using these materials. Complex compensation measures, such as specific objective designs with combinations of different birefringent lens materials or crystal orientations are therefore required in order to ensure adequate imaging quality of such projection objectives.
Further crystal materials which may be used in a projection objective for microlithography, such as lithium fluoride (LiF), barium fluoride (BaF2), potassium fluoride (KCl), sodium fluoride (NaCl) or sapphire (Al2O3) am mentioned in the article by G. Roblin, “Problèmes posés par la conception d'un objectif photoréducteur fonctionnant en UV” [Problems posed by the design of a photoreducing objective operating in the UV] from J. Optics (Pais), 1984, Vol. 15 No. 4 pages 281-285.
John H. Burnett et al., “High Index Materials for 193 nm and 157 nm Immersion Lithography” International Symposium on Immersion & 157 nm Lithography, Vancouver, Feb. 8, 2004 likewise mentions a range of materials for use in a projection objective for microlithography, in particular in an immersion objective, also including alkaline-earth metal-oxide single crystals such as magnesium oxide (Mgo) calcium oxide (CaO), strontium oxide (SrO) or barium oxide (BaO) as well as mixed crystals such as MgAl2O4 (magnesium spinel or spinel) or MgxCa1-xO. However, all of these materials exhibit considerable intrinsic birefringence (often referred to as IBR) even at 193 nm. For example, measurements of the IBR-dependent retardation for magnesium spinel have produced a value of 52 nm/cm for magnesium spinel at a wavelength of λ=193 nm. As a consequence very similar problems to those when using CaF2 arise.
A further difficulty with the use of single crystal material for optical elements is the production of the blanks, since the growth of single crystals is a highly elaborate process. The single crystal blanks generally have cylindrical symmetry, in particular even a cylindrical shape When manufacturing optical elements for a projection objective whose geometry often differs to a major extent from the cylindrical symmetry or cylindrical shape of the blanks, it is therefore generally necessary to remove a considerable amount of material. In addition to the loss of material associated with this, this also results in specific manufacturing problems, depending on the material characteristics, such as the hardness or the cleavability of the crystals used.
In addition to glasses and single crystals, polycrystalline solids are also known as optical materials. For example, U.S. Pat. No. 6,406,769 discloses the provision of a cover glass composed of polycrystalline spinel for wristwatches.
U.S. Pat. No. 5,536,244 discloses a closure window composed of single crystal or polycrystalline spinel for an endoscope.
In conjunction with recent developments in the field of nanoceramics, optically transparent nanocrystalline oxides have become known. For example, the article by G. D. West, J. M. Perkins and M. H. Lewis, “Transparent Fine-Grained Oxide Ceramics”, Key Engineering Materials, Vols. 264-268 (2004), pages 801-804 discloses especially for polycrystalline Al2O3 that the transparency increases as the grain size of the Al2O3 crystallites decreases.
A method for production of nanocrystalline MgAl2O4 with crystallite sizes of less than 100 nm is disclosed in the article by Xianghui Chang et al., “MgAl2O4 Transparent Nano-Ceramics Prepared by Sintering under High Pressure”, Key Engineering Materials, Vols. 280-283 (2005), pages 549-552.